If you are looking for a desired percentage of either CBD or THC by blending, there is a simple method called a Pearson Square. This is used for mixing livestock feed to a specific percent protein, but works for blending any 2 things to a desired ratio.
Instead on using soybean meal and corn’s percent CP (crude protein) you would use Strain A and Strain B with their percent CBD or THC. Place the desired percentage in the middle and simply subtract diagonally, and you get the mixing ratio in parts across the square.
Note that the desired percentage needs to be between the the 2 strains, and you ignore negatives.
You want THC to be exactly 4%.
Strain A is 5.5% CBD and 17% THC
Strain B is 9.2% CBD and 0.7% THC
17% - 4% = 13 parts (grams) Strain B
0.7% - 4% = 3.3 parts (grams) Strain A
So, if you blend 13g of B and 3.3g of A you will have a blend that is 4% THC.
CBD is pretty simple to find after that.
[(A grams * A%CBD] + (B grams * B%CBD)] / (grams A+B)
[(139.2) + (3.35.5)] / (13+3.3) = 8.45% CBD